This invention relates to gypsum products. More specifically, it relates to a gypsum-based panel that requires less time or less energy for drying than conventional products.
Gypsum-based panels are commonly used in construction. Wallboard made of gypsum is fire retardant and can be used in the construction of walls of almost any shape. It is used primarily as an interior wall or exterior wall or ceiling product. Gypsum has sound-deadening properties. It is relatively easily patched or replaced if it becomes damaged. There are a variety of decorative finishes that can be applied to the wallboard, including paint and wallpaper. Even with all of these advantages, it is still a relatively inexpensive building material.
One reason for the low cost of wallboard panels is that they are manufactured by a process that is fast and efficient. Calcium sulfate hemihydrate hydrates in the presence of water to form a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. A slurry that includes the calcium sulfate hemihydrate and water is prepared in a mixer. When a homogeneous mixture is obtained, the slurry is continuously deposited on a moving surface that optionally includes a facing material. A second facing material is optionally applied thereover before the slurry is smoothed to a constant thickness and shaped into a continuous ribbon. The continuous ribbon thus formed is conveyed on a belt until the calcined gypsum is set, and the ribbon is thereafter cut to form panels of desired length, which panels are conveyed through a drying kiln to remove excess moisture. Since each of these steps takes only minutes, small changes in any of the process steps can lead to gross inefficiencies in the manufacturing process.
The amount of water added to form the slurry is in excess of that needed to complete the hydration reaction. Excess water gives the slurry sufficient fluidity to flow out of the mixer and onto the facing material to be shaped to an appropriate width and thickness. As the product starts to set, the water pools in the interstices between dihydrate crystals. The hydration reaction continues building the crystal matrix in and around the pools of water, using some of the pooled water to continue the reaction. When the hydration reactions are complete, the unused water occupying the pools leaves the matrix by evaporation. Interstitial voids are left in the gypsum matrix when all water has evaporated. The interstitial voids are larger and more numerous where large amounts of excess water are used.
While the product is wet, it is very heavy to move and relatively fragile. The excess water is removed from the board by evaporation. If the excess water were allowed to evaporate at room temperature, it would take a great deal of space to stack and store wallboard while it was allowed to air dry over a relatively lengthy time period or to have a conveyor long enough to provide adequate drying time. Until the board is set and relatively dry, it is somewhat fragile, so it must be protected from being crushed or damaged.
To hasten evaporation, the wallboard panel is usually dried by evaporating the excess water at elevated temperatures, for example, in an oven or kiln. It is relatively expensive to operate the kiln at elevated temperatures, particularly when the cost of fossil fuels rises. A reduction in production costs could be realized by reducing the amount of excess water present in set gypsum boards that is later removed by evaporation.
Dispersants are known for use with gypsum that help fluidize the mixture of water and calcium sulfate hemihydrate so that less water is needed to make a flowable slurry.
β-Naphthalene sulfonate formaldehyde (“BNS”) and melamine sulfonate formaldehyde (“MFS”) condensate dispersants are well known, but have limited efficacy. The preparation and use of BNS is well known state of the art and disclosed in EP 0 214 412 A1 and DE-PS 2 007 603, herein incorporated by reference. The effect and properties of BNS can be modified by changing the molar ratio between formaldehyde and the naphthalene component that usually is from about 0.7 up to about 3.5. The ratio between formaldehyde and the sulfonated naphthalene component preferably is from about 0.8 to 3.5 to about 1. BNS condensates are added to the hydraulic binder containing composition in amounts from about 0.01 up to about 6.0 wt. %.
Melamine-sulfonate-formaldehyde-condensates are broadly used as flow improving agents in the processing of hydraulic binder containing compositions such as dry mortar mixtures, pourable mortars and other cement bonded construction materials and in the production of gypsum panels. Melamine is used in this connection as representative of s-triazine. They cause a strong liquefying effect of the construction chemicals mixture while minimizing undesired side effects occurring in the processing or in the functional properties of the hardened building material. As it is for the BNS technology, there is also broad prior art for MFS. MFS dispersants are revealed in DE 196 09 614 A1, DE 44 11 797 A1, EP 0 059 353 A1 and DE 195 38 821A1.
DE 196 09 614 A1 discloses a water soluble polycondensation product based on an amino-s-triazine and its use as plasticizer in aqueous binder containing suspensions based on cement, lime and gypsum. These polycondensates are capable in two condensation steps whereby in a pre-condensation step the amino-s-triazine, the formaldehyde component and the sulfite are condensed at a molar ratio of 1 to 0.5:5.0 to 0.1:1.5. Melamine is a preferred representative of amino-s-triazines. Further suitable representatives are amino plast former selected from the group urea, thiourea, dicyandiamide or guanidine and guanidine salts.
According to DE 44 11 797 A1 sulfanilic acid-containing condensation products based on amino-s-triazines that show at least two amino groups are prepared by using formaldehyde. The sulfanilic acid is used in amounts of from 1.0 to 1.6 mol per mol amino-s-triazine and neutralized in aqueous solution with an alkaline metal hydroxide or in earth alkaline metal hydroxide. In an additional step the formaldehyde is added in amounts of from 3.0 to 4.0 mol per mol amino-s-triazine at a pH value between 5.0 to 7.0 and at temperatures between 50 and 90° C. The final viscosity of the solution is between 10 and 60 cSt at 80° C.
According to EP 0 059 353 A1 highly concentrated and low viscosity aqueous solutions of melamine/aldehyde resins are capable by reacting melamine and an aldehyde in an alkaline medium in a first step with a component selected from the group comprising alkali sulphate, earth alkali sulphate or (earth) alkali sulfonate or other suitable amino compounds to a pre-condensate. This mixture in an additional process step is reacted with another amino compound such as amino acids or amino carbonic acids and finally the resin solution is brought to an alkaline pH value.
DE 195 38 821A1 discloses a condensate based on an amino-s-triazine with at least two amino groups and formaldehyde, and a high content of sulfonic acid groups and a low content of formate. Such products can be prepared according to this document by reacting the amino-s-triazine, formaldehyde and a sulfite at a molar ratio of 1:3.0:6.0:1.51:2.0 in an aqueous solution and at a temperature between 60 and 90° C. and a pH value between 9.0 and 13.0 until the sulfite is no longer present. In an additional step the condensation process is conducted at a pH value between 3.0 and 6.5 and at temperatures between 60 and 80° C. until the condensation product at 80° C. shows a viscosity between 5 and 50 mm2/s. Finally, the condensation product is to be brought to a pH value between 7.5 and 12.0 or treated thermally by a pH≧10.0 and a temperature between 60 and 100° C.
Polycarboxylate dispersants are commonly used with cements and, to a lesser degree, with gypsum. The class of compounds represented by the term “polycarboxylate dispersants” is large, and it is very difficult to predict how individual compounds react in different media. The use of a two-monomer polycarboxylate dispersant in gypsum products is disclosed in U.S. Pat. No. 7,767,019, herein incorporated by reference.
As has been previously disclosed, many polycarboxylate dispersants have deleterious effects on gypsum-based products. These dispersants retard setting of the calcined gypsum. The degree of retardation depends on the exact formulation of the polycarboxylate dispersant. Some polycarboxylate dispersants also cause a loss in compressive strength of foamed gypsum casts due to stabilization of foam. This leads to formation of smaller voids within the set gypsum. It is difficult to predict how severely a polycarboxylate dispersant will react in a gypsum slurry merely from the chemical formula.
A relatively new class of dispersants has become known for use in cements. It is a phosphated polycondensate dispersant. Although this dispersant is very effective for use in cement, it has lower efficacy in gypsum slurries compared to polycarboxylate dispersants, but it is also low in set retardation.
WO 2006/042709 describes polycondensates based on an aromatic or heteroaromatic compound (A) having 5 to 10 C atoms or heteroatoms, having at least one oxyethylene or oxypropylene radical, and an aldehyde (C) selected from the group consisting of formaldehyde, glyoxylic acid and benzaldehyde or mixtures thereof, which result in an improved plasticizing effect of inorganic binder suspensions compared with the conventionally used polycondensates and maintain this effect over a longer period (“slump retention”). In a particular embodiment, these may also be phosphated polycondensates. The phosphated monomers used are, however, relatively expensive since they have to be separately prepared and purified.
Alternatively, there has been developed an economical dispersant, based on a phosphated polycondensate, for hydraulic binders, which dispersant is particularly suitable as a plasticizer/water-reducing agent for concrete and can be prepared in a simple manner and at low cost. It is described in provisional application EP 081659155.3, filed in August 2008.
Those who install gypsum panels become fatigued by continuously moving and lifting the panels. It is, therefore advantageous to make panels that are lightweight for ease in handling. Lightweight panels can be made by adding foam to the gypsum slurry. A foaming agent, such as soap, can be added to the slurry so that foam is produced by the mixing action. In some cases, the foaming agent is used to pregenerate a foam that is added to the slurry after it exits the mixer. The foaming agent is selected to produce a foam that is actively coalescing while hydration is taking place. A distribution of foam bubble sizes results from an “active” foam. As the hydration reactions proceed, the gypsum matrix builds up around the foam bubbles, leaving foam voids in the matrix when the set gypsum forms and the foam bubbles break.
It can be difficult to obtain a distribution of foam voids that results in an acceptable panel strength. Foam voids that are very small and numerous have very thin walls of gypsum matrix between them. Poor compressive strength of the finished panel may result. Formation of very large foam voids can produce unevenness in the surface of the panel, making it aesthetically unacceptable. It has been found that when the set gypsum has a distribution of large and small foam voids, the panel can have both strength and an aesthetically pleasing appearance. This foam void distribution can be achieved by using a combination of soaps that form stable foam and soaps that form unstable foam.
It is clear that design of a gypsum panel includes many variables that are interrelated. Dispersants used to reduce water also change the set time of the gypsum slurry. Some dispersants stabilize foam bubbles, while other dispersants destabilize the foam. Set accelerators that decrease the initial hydration time also reduce initial fluidity of the slurry. In addition to changing bubble size distribution, soaps affect slurry fluidity. The additives used to control the slurry fluidity, hydration time and foam bubble size distribution each affect multiple variables, making it difficult to strike a balance among all of these factors.